In the following, portions of technical terms in an explanation are based on what are used in a Blu-ray Disc (BD). However, an application range of the present invention is not limited to BD.
An enlargement of a recording capacity of an optical disc have been realized by forming a short wavelength of a light source, increasing a numerical aperture (NA) of an objective lens, and increasing a number of recording layers per disc in addition thereto. BD realizes a recording capacity of 50 GB by two layers by using a blue semiconductor laser and a high NA objective lens with NA of 0.85. In 2010, BDXL having a recording capacity equal to or more than 100 GB was reduced into practice by increasing a number of recording layers to 3 or 4 and at the same time, also increasing an areal recording density.
Shortening of a recording wavelength and an increasing NA of objective lens are near to limits and in the future, it is not easy to considerably increase an areal recording density. Therefore, it is one of promising resolving means to further increase a number of recording layers in order to realize a recording capacity more than the above-described.
However, when a number of recording layers is going to be increased by a configuration similar to that of a multilayered optical disc of a background art, there is a high possibility that a reduction in cost per a recording capacity is difficult to be realized. That is because fabrication cost and yield of a current multilayered optical disc exclusively relate to a process of forming a recording layer. More specifically, an increase in a number of layers is directly linked to an increase in a number of steps, and a final yield is generally determined by a yield in stamping per one layer raised to the power of the number of layers.
Hence, there is investigated a system dispensing with stamping when a recording layer is formed as described in ISOM10 Technical Abstract, Th-L-07, “16 Layers Write Once Disc with a Separated Guide Layer”. The system is featured in that a layer that is formed with a groove used for tracking (hereinafter, referred to as reference layer) is provided other than respective recording layers, and tracking is carried out by using a beam other than a beam used for recording and reproducing. In the following, this is referred to as groove-less multilayer system.
Also, an investigation is carried out on an optical disc which does not have a recording layer that is physically defined as in a multilayered optical disc of a background art and a recording technology thereof. As an example, according to a technology described in Japanese Unexamined Patent Application Publication No. 2008-97723, a recording region consisting of a photorefractive material is recorded with a microhologram, that is, small interference fringes. There is not a structure of physically specifying a recording position in the recording region described above, and therefore, recording positions of respective microholograms are determined by indirectly controlling a focal point position of light (recording light) used for recording. Taking another example, recording is carried out by forming a void in a recording region as described in Japanese Unexamined Patent Application Publication No. 2009-238285. According to the recording methods, a virtual recording layer can be increased comparatively freely, and an increase in a recording capacity per one sheet of disc is easy to be achieved. Incidentally, in the present specification, the system which does not a have a layer that physically specifies a recording position in a recording region as described above is generally referred to as spatial recording for convenience of explanation. Even in this system, a reference layer is used for tracking.